Method and apparatus for distributed processing for wireless sensors

ABSTRACT

Certain aspects of the present disclosure relate to a method for compressed sensing (CS). The CS is a signal processing concept wherein significantly fewer sensor measurements than that suggested by Shannon/Nyquist sampling theorem can be used to recover signals with arbitrarily fine resolution. In this disclosure, the CS framework is applied for sensor signal processing in order to support low power robust sensors and reliable communication in Body Area Networks (BANs) for healthcare and fitness applications.

CLAIM OF PRIORITY

The present application for patent is a continuation application of, andclaims the benefit of priority from, U.S. patent application Ser. No.12/777,698 filed on May 11, 2010, which in turn claims priority toProvisional Application No. 61/266,324, filed on Dec. 3, 2009, bothapplications being assigned to the assignee hereof and expresslyincorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No.12/777,740, filed concurrently with U.S. patent application Ser. No.12/777,698, and entitled METHOD AND APPARATUS FOR DISTRIBUTED PROCESSINGFOR WIRELESS SENSORS, and U.S. patent application Ser. No. 13/532,078filed on Jun. 25, 2012 and entitled METHOD AND APPARATUS FOR DISTRIBUTEDPROCESSING FOR WIRELESS SENSORS, which is a divisional application ofU.S. patent application Ser. No. 12/777,740, both applications beingassigned to the assignee hereof and expressly incorporated herein byreference.

BACKGROUND

Field

Certain aspects of the present disclosure generally relate to signalprocessing and, more particularly, to a method for distributedprocessing for wireless sensors.

Description of Related Technology

Networks such as body area networks (BANs) make use of wireless sensorssuch as pulse oximeters to monitor vital signs of individuals. In orderto improve the performance of such BANs, it is desirable to reduce thepower consumption and complexity of such wireless sensors. Thus there isa need for low power sensors which can accurately detect and communicateinformation such as vital signs of individuals.

SUMMARY

In one aspect, a method of processing data is provided. The methodincludes maintaining, at a first apparatus, a first clock signal,transmitting, from the first apparatus, a clock synchronization signalfor synchronizing a second clock signal, at a second apparatus, to thefirst clock signal, receiving, at the first apparatus, one or morepackets comprising a first set of samples indicative of an input over aperiod of time, the first set of samples corresponding to the secondclock signal, and generating, at the first apparatus, a second set ofsamples based, at least in part, on the first set of samples, whereinthe second set of samples corresponds to the first clock signal.

In another aspect, an apparatus for processing data is provided. Theapparatus includes a processing system, configured to maintain a firstclock signal, a transmitter configured to transmit a clocksynchronization signal for synchronizing a second clock signal to thefirst clock signal, and a receiver configured to receive one or morepackets comprising a first set of samples indicative of an input over aperiod of time, the first set of samples corresponding the second clocksignal. The processing system is further configured to generate a secondset of samples based, at least in part, on the first set of samples, thesecond set of samples corresponding to the first clock signal.

Another aspect provides an apparatus for processing data. The apparatusincludes means for maintaining a first clock signal, means fortransmitting a clock synchronization signal for synchronizing a secondclock signal to the first clock signal, means for receiving one or morepackets comprising a first set of samples indicative of an input over aperiod of time, the first set of samples corresponding the second clocksignal, and means for generating a second set of samples based, at leastin part, on the first set of samples, the second set of samplescorresponding to the first clock signal.

Another aspect provides a computer program product. The computer programproduct includes a computer-readable medium. The computer-readablemedium includes computer executable instructions that, if executed by anapparatus, cause the apparatus to perform a method. The method includesmaintaining, a first clock signal, transmitting a clock synchronizationsignal for synchronizing a second clock signal to the first clocksignal, receiving one or more packets comprising a first set of samplesindicative of an input over a period of time, the first set of samplescorresponding to the second clock signal, and generating a second set ofsamples based, at least in part, on the first set of samples, whereinthe second set of samples corresponds to the first clock signal.

Another aspect provides a mobile phone. The mobile phone includes anantenna, a processing system configured to maintain a first clocksignal, a transmitter configured to transmit, via the antenna, a clocksynchronization signal for synchronizing a second clock signal to thefirst clock signal, and a receiver configured to receive, via theantenna, one or more packets comprising a first set of samplesindicative of an input over a period of time, the first set of samplescorresponding the second clock signal. The processing system is furtherconfigured to generate a second set of samples based, at least in part,on the first set of samples, the second set of samples corresponding tothe first clock signal.

Another aspect provides a method of data communication. The methodincludes receiving a clock synchronization signal, adjusting a clocksignal based, at least in part, on the clock synchronization signal,generating a first set of samples corresponding to the adjusted clocksignal, generating one or more packets comprising, at least in part, thefirst set of samples, and transmitting the one or more packets.

Another aspect provides an apparatus for data communication. Theapparatus includes a receiver configured to receive a clocksynchronization signal, a processing system configured to adjust a clocksignal based, at least in part, on the clock synchronization signal,generate a first set of samples corresponding to the adjusted clocksignal, and generate one or more packets comprising, at least in part,the first set of samples, and a transmitter configured to transmit theone or more packets.

Another aspect provides an apparatus for data communication. Theapparatus includes means for receiving a clock synchronization signal,means for adjusting a clock signal based, at least in part, on the clocksynchronization signal, means for generating a first set of samplescorresponding to the adjusted clock signal, means for generating one ormore packets comprising, at least in part, the first set of samples, andmeans for transmitting the one or more packets.

Another aspect provides a computer program product. The computer programproduct includes a computer-readable medium. The computer-readablemedium includes computer executable instructions that, if executed by anapparatus, cause the apparatus to perform a method. The method includesreceiving a clock synchronization signal, adjusting a clock signalbased, at least in part, on the clock synchronization signal, generatinga first set of samples corresponding to the adjusted clock signal,generating one or more packets comprising, at least in part, the firstset of samples, and transmitting the one or more packets.

Another aspect provides a sensing device. The device includes atransducer configure to receive an input, a receiver configured toreceive a clock synchronization signal, a processing system configuredto adjust a clock signal based, at least in part, on the clocksynchronization signal, generate a first set of samples corresponding tothe adjusted clock signal based, at least in part, on the input, andgenerate one or more packets comprising, at least in part, the first setof samples, and a transmitter configured to transmit the one or morepackets.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description,briefly summarized above, may be had by reference to aspects, some ofwhich are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only certain typicalaspects of this disclosure and are therefore not to be consideredlimiting of its scope, for the description may admit to other equallyeffective aspects.

FIG. 1 illustrates an example wireless communication system.

FIG. 2 is a block diagram illustrating various components that may beutilized in a wireless device.

FIG. 3 is a block diagram of portions of a sensor in accordance withcertain aspects of the present disclosure.

FIG. 4 illustrates an example of a body area network (BAN).

FIG. 5 is a block diagram illustrating an array of sensors used withinthe BAN.

FIG. 6 is a block diagram illustrating an aggregator used within theBAN.

FIG. 7 illustrates an example of a time-domain photoplethysmograph (PPG)signal and its frequency spectrum.

FIG. 8 illustrates an example of a time-domain electro-cardiogram (ECG)signal and its frequency spectrum.

FIG. 9 illustrates a compressed sensing sample distribution.

FIG. 10 is a flowchart illustrating a method of sensing data.

FIG. 11 is a flowchart illustrating a method of processing data.

FIG. 12A is a flowchart illustrating a method for handling clockmisalignment.

FIG. 12B is a flowchart illustrating another method for handling clockmisalignment.

FIG. 13 illustrates an example block diagram of a sensor and are-constructor in accordance with certain aspects of the presentdisclosure.

FIG. 14A-B illustrates an example of a PPG signal and its transform inthe Gabor space in accordance with certain aspects of the presentdisclosure.

FIG. 15 illustrates example operations for under-sampled acquisition andreconstruction in accordance with certain aspects of the presentdisclosure.

FIG. 16 illustrates example operations for actuating and de-actuating asource at the sensor in accordance with certain aspects of the presentdisclosure.

FIG. 17 illustrates a comparison of reconstructed signals obtained usingdifferent under-sampling ratios (USRs) in accordance with certainaspects of the present disclosure.

FIG. 18 illustrates an example of a signal sensed at non-uniformsampling instances in accordance with certain aspects of the presentdisclosure.

FIG. 19A-C illustrate waveforms depicting the effect of a clock mismatchbetween a transmitter and receiver with transmitter clock rate higherthan receiver clock rate in accordance with certain aspects of thepresent disclosure.

FIG. 20A-C illustrate additional waveforms depicting the effect of aclock mismatch between a transmitter and receiver with transmitter clockrate lower than receiver clock rate in accordance with certain aspectsof the present disclosure.

FIG. 21A-B illustrate waveforms depicting the effect of omitting a lowpass filter under Nyquist sampling and compressed sensing regimes inaccordance with certain aspects of the present disclosure.

FIG. 22 is a block diagram of an apparatus in accordance with certainaspects of the present disclosure.

FIG. 23 is a block diagram of another apparatus in accordance withcertain aspects of the present disclosure.

DETAILED DESCRIPTION

Various aspects of the novel systems, apparatus and methods aredescribed more fully hereinafter with reference to the accompanyingdrawings. The teachings disclosure may, however, be embodied in manydifferent forms and should not be construed as limited to any specificstructure or function presented throughout this disclosure. Rather,these aspects are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the disclosure to thoseskilled in the art. Based on the teachings herein one skilled in the artshould appreciate that that the scope of the disclosure is intended tocover any aspect of the novel systems, apparatus, and methods disclosedherein, whether implemented independently of or combined with any otheraspect of the invention. For example, an apparatus may be implemented ora method may be practiced using any number of the aspects set forthherein. In addition, the scope of the invention is intended to coversuch an apparatus or method which is practiced using other structure,functionality, or structure and functionality in addition to or otherthan the various aspects of the invention set forth herein. It should beunderstood that any aspect disclosed herein may be embodied by one ormore elements of a claim.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any aspect described herein as “exemplary”is not necessarily to be construed as preferred or advantageous overother aspects.

Although particular aspects are described herein, many variations andpermutations of these aspects fall within the scope of the disclosure.Although some benefits and advantages of the preferred aspects arementioned, the scope of the disclosure is not intended to be limited toparticular benefits, uses, or objectives. Rather, aspects of thedisclosure are intended to be broadly applicable to different wirelesstechnologies, system configurations, networks, and transmissionprotocols, some of which are illustrated by way of example in thefigures and in the following description of the preferred aspects. Thedetailed description and drawings are merely illustrative of thedisclosure rather than limiting, the scope of the disclosure beingdefined by the appended claims and equivalents thereof.

An Example Wireless Communication System

The teachings herein may be incorporated into (e.g., implemented withinor performed by) a variety of wired or wireless apparatuses (e.g.,nodes). In some aspects, a node implemented in accordance with theteachings herein may comprise an access point or an access terminal.

An access point (“AP”) may comprise, be implemented as, or known asNodeB, Radio Network Controller (“RNC”), eNodeB, Base Station Controller(“BSC”), Base Transceiver Station (“BTS”), Base Station (“BS”),Transceiver Function (“TF”), Radio Router, Radio Transceiver, BasicService Set (“BSS”), Extended Service Set (“ESS”), Radio Base Station(“RBS”), or some other terminology.

An access terminal (“AT”) may comprise, be implemented as, or known asan access terminal, a subscriber station, a subscriber unit, a mobilestation, a remote station, a remote terminal, a user terminal, a useragent, a user device, user equipment, or some other terminology. In someimplementations an access terminal may comprise a cellular telephone, acordless telephone, a Session Initiation Protocol (“SIP”) phone, awireless local loop (“WLL”) station, a personal digital assistant(“PDA”), a handheld device having wireless connection capability, orsome other suitable processing device connected to a wireless modem.Accordingly, one or more aspects taught herein may be incorporated intoa phone (e.g., a cellular phone or smart phone), a computer (e.g., alaptop), a portable communication device, a portable computing device(e.g., a personal data assistant), an entertainment device (e.g., amusic or video device, or a satellite radio), a global positioningsystem device, or any other suitable device that is configured tocommunicate via a wireless or wired medium. In some aspects the node isa wireless node. Such wireless node may provide, for example,connectivity for or to a network (e.g., a wide area network such as theInternet or a cellular network) via a wired or wireless communicationlink.

FIG. 1 illustrates an example of a wireless communication system 100 inwhich aspects of the present disclosure may be employed. The wirelesscommunication system 100 may be a broadband wireless communicationsystem. The wireless communication system 100 may provide communicationfor a number of cells 102, each of which is serviced by a base station104. A base station 104 may be a fixed station that communicates withuser terminals 106. The base station 104 may alternatively be referredto as an access point, a Node B or some other terminology.

FIG. 1 depicts various user terminals 106 dispersed throughout thesystem 100. The user terminals 106 may be fixed (i.e., stationary) ormobile. The user terminals 106 may alternatively be referred to asremote stations, access terminals, terminals, subscriber units, mobilestations, stations, user equipment, etc. The user terminals 106 may bewireless devices, such as cellular phones, personal digital assistants(PDAs), handheld devices, wireless modems, laptop computers, personalcomputers, etc.

A variety of processes and methods may be used for transmissions in thewireless communication system 100 between the base stations 104 and theuser terminals 106. For example, signals may be sent and receivedbetween the base stations 104 and the user terminals 106 in accordancewith OFDM/OFDMA techniques. If this is the case, the wirelesscommunication system 100 may be referred to as an OFDM/OFDMA system.Alternatively, signals may be sent and received between the basestations 104 and the user terminals 106 in accordance with CDMAtechnique. If this is the case, the wireless communication system 100may be referred to as a CDMA system.

A communication link that facilitates transmission from a base station104 to a user terminal 106 may be referred to as a downlink (DL) 108,and a communication link that facilitates transmission from a userterminal 106 to a base station 104 may be referred to as an uplink (UL)110. Alternatively, a downlink 108 may be referred to as a forward linkor a forward channel, and an uplink 110 may be referred to as a reverselink or a reverse channel.

A cell 102 may be divided into multiple sectors 112. A sector 112 is aphysical coverage area within a cell 102. Base stations 104 within awireless communication system 100 may utilize antennas that concentratethe flow of power within a particular sector 112 of the cell 102. Suchantennas may be referred to as directional antennas.

FIG. 2 illustrates various components that may be utilized in a wirelessdevice 202 that may be employed within the wireless communication system100. The wireless device 202 is an example of a device that may beconfigured to implement the various methods described herein. Thewireless device 202 may be a base station 104 or a user terminal 106.

The wireless device 202 may include a processor 204 which controlsoperation of the wireless device 202. The processor 204 may also bereferred to as a central processing unit (CPU). Memory 206, which mayinclude both read-only memory (ROM) and random access memory (RAM),provides instructions and data to the processor 204. A portion of thememory 206 may also include non-volatile random access memory (NVRAM).The processor 204 typically performs logical and arithmetic operationsbased on program instructions stored within the memory 206. Theinstructions in the memory 206 may be executable to implement themethods described herein. Further, the functionality of processor 204and the DSP 220 may be included in a single module.

The wireless device 202 may also include a housing 208 that may includea transmitter 210 and a receiver 212 to allow transmission and receptionof data between the wireless device 202 and a remote location. Thetransmitter 210 and receiver 212 may be combined into a transceiver 214.An antenna 216 may be attached to the housing 208 and electricallycoupled to the transceiver 214. The wireless device 202 may also include(not shown) multiple transmitters, multiple receivers, multipletransceivers, and/or multiple antennas.

The wireless device 202 may also include a signal detector 218 that maybe used in an effort to detect and quantify the level of signalsreceived by the transceiver 214. The signal detector 218 may detect suchsignals as total energy, energy per subcarrier per symbol, powerspectral density and other signals. The wireless device 202 may alsoinclude a digital signal processor (DSP) 220 for use in processingsignals. The signal detector may also be connected to a transducer suchas a microphone, ECG electrodes, photodiodes, etc (not shown).

The various components of the wireless device 202 may be coupledtogether by a bus system 222, which may include a power bus, a controlsignal bus, and a status signal bus in addition to a data bus.

Body Area Network Concept

FIG. 4 illustrates an example of a body area network (BAN) 400. Bodyarea networks represent a promising concept for healthcare applicationssuch as continuous monitoring of vital signs for diagnostic purposes,effects of medicines on chronic ailments, etc.

The BAN may consist of several acquisition circuits. Each acquisitioncircuit may comprise wireless sensor that senses one or more vital signsand communicates them to an aggregator (i.e., an access terminal) suchas a mobile handset, a wireless watch, or a Personal Data Assistant(PDA). The aggregator is sometimes referred to as the gateway. Sensors402, 404, 406, and 408 that acquire various biomedical signals andtransmit them over a wireless channel to an aggregator 410 may have thesame functionality as access points 104. FIG. 5 illustrates detailedblock diagram of an array of biomedical sensors 501 and 550 that maycorrespond to sensors 402-408 within the BAN 400.

The aggregator 410 illustrated in FIG. 4 may receive and process variousbiomedical signals transmitted over a wireless channel from sensors402-408. The aggregator 410 may be a mobile handset or a PDA, and mayhave the same functionality as a mobile device 106 from FIG. 1. In otherembodiments, the aggregator 410 may be an access point or femto node.FIG. 6 illustrates a detailed block diagram of an aggregator 610 thatmay correspond to the aggregator 410 within the BAN 400. The aggregator610 may be an example of the receiver 212 from FIG. 2.

It may be desirable for sensors used in the BAN to be non-intrusive andlong lasting. Photoplethysmograph (PPG) and Electro Cardiogram (ECG)signals may be considered in this disclosure to demonstrate benefits ofcompressed sensing (CS) techniques for sensor signal processing. ThePPG, the ECG and the activity sensing cover a large percentage ofchronic ailments in a large segment of human population, and thusprovide significant opportunities for wireless technologies in the BANand mobile devices with wireless area network (WAN) connectivity toimprove diagnosis and care of such ailments.

Pulse oximeter sensors can generate the PPG waveform which may enablecontinuous monitoring of blood oxygenation (also called S_(p)O₂), acrucial indicator of pulmonary system including lungs and respiration. Ablood carries oxygen, nutrients and chemicals to the body cells in orderto ensure their survival, proper functioning and to remove the cellularwastes. S_(p)O₂ is extensively used in clinical settings for diagnosis,surgery, long term monitoring, etc. FIG. 7 illustrates an example of atime-domain PPG signal and its frequency spectrum.

The ECG is another important vital sign for assessing the cardiovascularsystem. Heart is one of the most hard working body parts, pumping aboutsix liters of blood per minute through the body in humans. Electricalsignals generated during each cardiac cycle form the ECG signal and canbe easily captured by Ag/AgCl electrode sensors. The ECG may beroutinely used in clinical settings for diagnosing heart relatedproblems and continuous monitoring of the ECG may enable early diagnosisof many chronic conditions. FIG. 8 illustrates an example of atime-domain ECG signal and its frequency spectrum. A blood pressure (BP)is another vital sign with enormous clinical value. A systolic bloodpressure (SBP) and a diastolic blood pressure (DBP) may be estimatedusing the ECG and the PPG signals.

In certain aspects, the sensors and gateways described herein make useof compressed sensing (CS). In CS, significantly fewer sensormeasurements than that suggested by Shannon/Nyquist sampling theorem canbe used to recover signals with arbitrarily fine resolution. This ispossible when the signals being sensed are inherently compressible orsparse in certain domain. A class of band-limited signals with Mnon-zero spectral components is considered, where M<<f_(s)/2 and f_(s)is a suggested sampling rate, such as the Nyquist sampling rate.Traditionally, such signals can be compressed after acquisition for moreefficient transmission and/or storage.

In the CS framework, the acquisition process (i.e., sensing) can beintegral to source compression and can be independent of the sparsenature of the signals. However, this sparsity information may berequired at a receiver side in order to perform signal reconstruction.Measurements in the CS framework are generally defined as inner productsof the signal with random basis functions. These signals can beaccurately recovered if at least M log(N/M) samples are available at thereceiver, where N is the number of samples in the reconstruction, albeitwith some additional computational complexity at the receiver. This canbe useful in the context of a body area network (BAN) as thecomputational complexity is shifted to nodes with flexible power budgetin order to increase working life of sensors employed in the BAN.

The CS paradigm can be used for applications concerning signaldetection/classification, imaging, data compression and MagneticResonance Imaging (MRI). Benefits of the CS are reported in terms ofimproved signal fidelity and superior recognition performance In thepresent disclosure, the CS-based signal processing is proposed forproviding low power sensors within the BAN for healthcare and fitnessapplications.

One aspect of the BAN in healthcare applications is to provide areliable communication link between sensors (i.e., transmitters) and anaggregator (i.e., a receiver), while minimizing sensor power andcommunication latency.

FIG. 5 illustrates an example block diagram of array of sensors usedwithin the BAN in accordance with certain aspects of the presentdisclosure. In one aspect, sensors 501 and 550 are designed to takeadvantage of compressed sensing in order to significantly lower thepower consumption and complexity of the circuitry used in the sensor.For example, the use of compressed sensing may allow the sensors 501 and550 to omit certain processing steps and certain signal conditioningsteps in order to decrease power consumption. The omission of thesesteps and the corresponding circuitry and functionality is described ingreater detail below.

Sensor 501 comprises processing system 505. Processing system 505 mayoperate to control and direct the operation of sensor 501. In oneaspect, processing system 505 includes clock circuitry for generating,maintaining, and adjusting a clock signal. As described herein, thereduced complexity of the operation of sensor 501 may allow processingsystem 505 to be simplified such that more power intensive hardware suchas a CPU may not be necessary for the sensor 501 to perform itsfunction. For example, a dedicated hardware finite state machine couldbe used instead. Processing system 505 is coupled to a digital-to-analogconverter (DAC) 508. The DAC 508 is coupled to a transducer 509. The DAC508 may be configured to drive the transducer 509 in order to generatesignals to be measured by the sensor 501. For example, in one aspect,sensor 501 comprises a pulse oximeter sensor. The sensor 501 may use DAC508 to convert an actuation signal from the processing system 505 inorder to drive transducer 509, a light emitting diode (LED) in thisaspect, for measurement purposes. In other aspects, sensors may notrequire actuation signals and may omit DAC 508 and the transducer 509.The sensor 501 may further comprise a transducer 510 for receiving ananalog signal. As described above, the transducer 510 may comprise alight detector or other sense circuit. The transducer 510 is coupled toconditioning circuitry, such as, for example, low noise amplification(LNA) circuit 511. The LNA circuit 511 is coupled to a sampling circuitsuch as, for example, compressed sensing (CS) analog to digitalconverter (ADC) 513. In one aspect, the CS ADC 513 may be implemented,at least in part, as a sample and hold circuit (not shown) in line witha quantization circuit (not shown). The structure of CS ADC 513according to various aspects is described in greater detail below withrespect to FIG. 3. As described herein, the CS ADC 513 may be used togenerate a set of samples representing the analog signal received by thetransducer over a certain period of time. The sensor 501 may furthercomprise a media access controller 516 and radio 514. Processing system505, in conjunction with MAC 516, may be configured to packetize the setof samples generated by the CS ADC 513 and to transmit one or more ofsuch packets via the radio 514.

Advantageously, the reduced complexity of the operations at the sensor501 enables the elimination of certain circuitry that would otherwisetraditionally be used in sensors. For example, to perform sampling atthe Nyquist rate, an anti-aliasing filter is traditionally used toprevent aliasing. Such filtering may require significant powerconsumption. By using CS techniques, the performance requirements ofanti-aliasing circuitry may be relaxed. Alternatively, in someembodiments, the anti-aliasing circuitry may be omitted. This isillustrated with respect to FIGS. 21A and 21B. In FIG. 21A a Nyquistedge of 200 Hz is presented. FIG. 21A also illustrates a signal having asignificant component 2103 below the Nyquist edge, at about 37 Hz and asignificant component 2105 above the Nyquist edge at about 500 Hz. Thecomponent 2105 above the Nyquist edge is an out of band tone.Traditionally, a Nyquist edge of 200 Hz requires a Nyquist samplingfrequency of 400 samples/second. Traditional Nyquist sampling requiresan anti-aliasing filter to remove all components above 200 Hz and thensampling uniformly with 400 samples/second. The result of omitting theanti-aliasing filter but still using Nyquist reconstruction isillustrated in FIG. 21 B. In particular, waveform 2109 illustrates thespectrum of a digital signal uniformly sampled with 400 samples/second,but with no anti-aliasing filter. The aliasing effect of the 500 Hzcomponent 2105 of FIG. 21A is illustrated by the spurious component 2113at 100 Hz in FIG. 21B. Thus, as illustrated, the omission of ananti-aliasing filter while using traditional sampling techniquesprovides inadequate results. To contrast, waveform 2111 illustrates theresult of traditional Nyquist sampling when using an anti-aliasingfilter. As illustrated, the component 2103 at 37 Hz is preserved and nospurious tones are promulgated when traditional Nyquist sampling iscombined with an anti-aliasing filter. Further, FIG. 21B illustrates awaveform 2107 showing a reconstruction using CS sampling without ananti-aliasing filter. As illustrated, the waveform 2107 recreates thecomponent 2103 at 37 Hz but does not include any spurious tones fromaliasing. In generating the CS reconstruction 2107, the average samplingrate was 400 samples/second, which is the same as that for the Nyquistcases. However, as describe above, the sampling in the CS reconstructionis non-uniform. Further, a priori information that spectral componentsgreater than 100 Hz are out of band of interest may be utilized and outof band components may not be reconstructed.

In one aspect, additional sensors such as the sensor 550 may also beincluded in a body area network as described in FIG. 4. For example,sensor 501 may comprise a pulse oximeter while sensor 550 may comprise athree dimensional accelerometer. As described below, the output from thesensors 501 and 550 may be synthesized and otherwise processed by acommon gateway.

The processing system 505 may be implemented with one or moreprocessors. The one or more processors may be implemented with anycombination of general-purpose microprocessors, microcontrollers,digital signal processors (DSPs), field programmable gate array (FPGAs),programmable logic devices (PLDs), controllers, state machines, gatedlogic, discrete hardware components, dedicated hardware finite statemachines, or any other suitable entities that can perform calculationsor other manipulations of information.

The processing system 505 may also include machine-readable media forstoring software. Software shall be construed broadly to mean any typeof instructions, whether referred to as software, firmware, middleware,microcode, hardware description language, or otherwise. Instructions mayinclude code (e.g., in source code format, binary code format,executable code format, or any other suitable format of code). Theinstructions, when executed by the one or more processors, cause theprocessing system 505 to perform the various functions described herein.

FIG. 3 illustrates a low noise amplifier (LNA) 2219, a random delaygenerator 2220, a sample and hold (S&H) circuit 2221, and a quantizationcircuit 2223. The LNA circuit 2219 may correspond to the LNA circuit 511of FIG. 5. The random delay generator 2220, S&H circuit 2221 andquantization circuit 2223 may correspond to the CS ADC 513 of FIG. 5. Asillustrated, an analog signal X(t) may be received at the LNA 2219. Theanalog signal may be processed sequentially by the LNA circuit, 2219,the S&H circuit 2221, and the quantization circuit 2223. The randomdelay generator 2220 may control the instances at which the signal issampled by S&H circuit 2221 and sampled by quantization circuit 2223.The random delay generator 2220 may receive a seed as input for use ingenerating the non-uniform sampling instances. The output of thequantization circuit 2223 is a digital signal used advantageously in themanner described herein. In one aspect, each circuit is configured toprovide an output directly to the next circuit without any interveningcircuitry. Thus, as described above, in this aspect, no low pass oranti-aliasing filter is present or used. As described, this omissionadvantageously results in reduced power consumption, reduced complexity,and reduced cost for the sensor without compromising the quality of thedata provided by the sensor.

FIG. 6 illustrates an aggregator 610. Aggregator 610 may also bereferred to herein as a gateway 610. Gateway 610 may be used to receivedata, such as sets of samples from sensor inputs, from one or moresensors such as sensors 501 and 550 of FIG. 5. Gateway 610 may befurther configured to perform additional processing on the data asdescribed herein. In one aspect, gateway 610 comprises a BAN radio 613.The BAN radio 613 may be configured to receive transmissions fromsensors such as sensors 501 and 550 of FIG. 5. The BAN radio 613 mayalso be configured to transmit messages to one or more wireless sensors.For example, as described herein, gateway 610 may be configured totransmit requests for data or for clock synchronization to one or morewireless sensors via the BAN radio 613. Gateway 610 further comprises amedia access controller (MAC) 616 coupled to the BAN radio 613 forcontrolling communications through the BAN radio 613. Gateway 610further comprises a processing system 619 coupled to the MAC 616. Theprocessing system 619 may be configured to communicate with the sensorsvia the MAC 616 and BAN radio 613. For example, as described herein, theprocessing system 619 may be configured to receive a set of CS samplesfrom a sensor and to reconstruct a Nyquist set of samples from thereceived sample set. Similarly, the processing system 619 may mitigatepacket loss, remove artifacts in the received sample set, determine andmitigate clock misalignment between the gateway 610 and the sensors,align samples sets from a plurality of sensors, and analyze the receivedor reconstructed samples to interpret vital signs and trigger alarms. Inone aspect, processing system 619 includes clock circuitry forgenerating and maintaining a clock signal.

The gateway 610 may further comprise additional radios such as wide areanetwork radios 622, e.g., a third generation radio, or LAN radios 625,e.g., WiFi radios. These radios may be used to communicate informationfrom the sensors to other devices. For example, a person's cell phonemay function as the gateway 610. The gateway 610 may further compriseanother media access controller (MAC) 623 for controlling communicationbetween the processing system 619 and the WAN and LAN radios 622 and625. In some embodiments, the MAC's 616 and 623 may be implemented byshared hardware or software. After determining vital signs based on datafrom the sensors, the processing system 619 may transmit informationabout the vital signs to a remotely located physician via the WAN radios622 of the LAN radios 625. Advantageously, the centralized processing ofsamples from the sensors at the gateway 610 facilitates lower powerconsumption and reduced complexity in the sensors. Further, the gateway610 may act as a centralized clock synchronization controller for theplurality of sensors.

The processing system 619 may be implemented with one or moreprocessors. The one or more processors may be implemented with anycombination of general-purpose microprocessors, microcontrollers,digital signal processors (DSPs), field programmable gate array (FPGAs),programmable logic devices (PLDs), controllers, state machines, gatedlogic, discrete hardware components, dedicated hardware finite statemachines, or any other suitable entities that can perform calculationsor other manipulations of information.

The processing system 619 may also include machine-readable media forstoring software. Software shall be construed broadly to mean any typeof instructions, whether referred to as software, firmware, middleware,microcode, hardware description language, or otherwise. Instructions mayinclude code (e.g., in source code format, binary code format,executable code format, or any other suitable format of code). Theinstructions, when executed by the one or more processors, cause theprocessing system 619 to perform the various functions described herein.

FIG. 9 illustrates a compressed sensing sample distribution inaccordance with certain aspects of the present disclosure. As describedherein, for CS sampling, the following conditions may be met for theanalog signal to be faithfully reconstructed from the digital signal.First, the hold time T_(h)<½*f_(max), where f_(max) is the maximumspectral component in a signal x(t) with a magnitude ∥X(f_(max))∥<6*qdB, where q is the number of bits in quantizing x(n). Second, in anygiven time window, the number of measurements, k, in CS sampling shallbe c*M*log(N/M), where M is the number of spectral components above6*qdB and N is the number of Nyquist samples to be reconstructed in thegiven time window. The constant c may represent the coherence betweenthe measurement basis and the sparse basis. For example, the moreincoherent the two bases are, the smaller the value of c is. In certainaspects, c is selected so that k is approximately three to five timesthe value M. In some aspects c may have a value between 1 and 2. As canbe seen in FIG. 9, the number of samples in the same time-window is muchless than would be present for Nyquist rate sampling. In this example,an under-sampling ratio of 10 was used; this implies that on average,the number of samples used for a given time window for CS reconstructionis 1/10th that of traditional Nyquist sampling. Advantageously, usingfewer samples reduces the power consumption at the sensor. Indeed, lessprocessing is used to generate the samples. In addition, for suchsensors as pulse oximeters, LEDs or similar elements may be powered fora reduced duration corresponding to the sampling periods for the signal.When x(t) is sparse in the spectral domain, k may be much less thanF_(s) measurements for every second of observation, depending on theredundancy in x(t). In addition, there may be significant power gains inthe sensor from transferring analog circuit complexity at the sensor todigital domain reconstruction complexity at the gateway. As noted abovek, the number of measurements in a window, is determined in relationshipto M, the number of spectral components above 6*qdB. Thus, the number ofmeasurements takes out of band components above 6*qdB in toconsideration, even though they may not be reconstructed at the gateway.In practice, many real world sensors have reduced sensitivity at higherfrequencies and the increase in number of measurements due to out ofband signals may not be prohibitive. CS sampling and reconstruction isalso described in greater detail below.

Certain aspects of the present disclosure relates to methods forreducing power consumption of a pulse oximeter sensor. Commercial pulseoximeters may typically consume power in the order of 20-60 mW. The redand infrared light-emitting diodes (LEDs) account for most of thispower. A power-efficient design for a PPG sensor may bring the powerconsumption down to 1.5 mW. The duty cycle associated with LED lightingmay be reduced for a given uniform sampling rate. Fast detectors andhigher clock frequencies may be used among other optimizations. Thus,the LED may be switched on for T·f_(s) duration, where f_(s) and Trepresent the sampling rate and duration of lighting used to acquireeach sample, respectively.

The PPG signals may be sparse in a spectral domain, and hencecompressible. This may enable the usage of a compressed sensing (CS)framework in order to acquire the PPG signals. The PPG signals may besampled at non-uniform (i.e., random) time intervals, but with anaverage sampling rate of F_(s). In the CS approach, the sampling rateF_(s) may be much smaller than the uniform sampling rate f_(s). A factorf_(s)/F_(s) may be referred as an under-sampling ratio (USR). It can benoted that this sampling approach may result in a reduced powerconsumption (i.e., approximately by a factor of USR) of pulse oximetersensors used for PPG acquisition, as the LED may be lit up for onlyT·f_(s)/USR duration instead of T·f_(s).

A benefit of the CS-based approach compared to low pass filtering andsampling at f_(s)/USR is that the signal content above f_(s)/USR may notbe lost. Similarly, narrow-band signals at higher frequencies may alsobe acquired with a high USR. Another benefit of utilizing the CSframework is that the measurements may be independent of the transformspace used at reconstruction, including the Fourier space as intraditional Nyquist rate sampling.

FIG. 10 illustrates a method 1001 of sensing data in accordance withcertain aspects of the present disclosure. In one aspect, the method1001 may be implemented in a sensor such as sensor 501 of FIG. 5. Instep 1005, the sensor 501 receives a request for data corresponding to aperiod of time. This request may come from the gateway 610.Alternatively, the sensor 501 may be configured to periodically generatedata in response to other stimuli. Continuing at step 1027, the sensor501 determines whether or not an actuation signal is used in theparticular measurement being performed. For, example, if the sensor 501is detecting light that is generated by an LED, such as the LED 509, anactuation signal may be used to drive the LED. In this case, the methodproceeds to step 1010 to generate an actuation signal and acquireadditional samples. If no actuation signal is used for the particularmeasurement, the method proceeds to step 1015 to acquire additionalsamples. Continuing to step 1010, after receiving the request, thesensor 501 may generate an actuation signal. As described above, certainsensors may comprise LEDs or other circuits which are actuated in orderto generate a signal that is measured by the sensor 501. As describedabove, other sensors, such as accelerometers may not require actuationsignals and may omit this step. Continuing to step 1015, the sensor 501receives an analog signal. The analog signal may be indicative of somevital sign or other characteristic of a person. Continuing to step 1020,the sensor 501 may sample the analog signal. In one aspect, compressedsensing is used to generate the samples.

Continuing to decision step 1025, the sensor 501 determines if asufficient number of samples have been obtained so that the sampledsignal may be accurately reproduced by the gateway 610. As describedabove, some number k, of samples is determined to be sufficient toaccurately reconstruct the sampled signal. However, because of clockmisalignment, packet losses, or other issues, additional samples may beincluded or a number of samples may be omitted. This number ofadditional or absent samples may be referred to as δ. As describedherein, the gateway is able to handle reconstruction of the sampledsignal when receiving k+δ samples. If the number of samples isdetermined to be insufficient, the method returns to decision step 1027.However, if the number of samples is sufficient, the method proceeds tostep 1030. At step 1030, the sensor 1030 generates one or more packetscontaining the samples of the analog signal. Continuing at step 1035,the packets are transmitted to the gateway 610. Advantageously, thereduced number of samples acquired at the sensor and the reduced processperformed on the sampled signal results in reduced power consumption andcircuit complexity at the sensor.

FIG. 11 illustrates a method 1101 of processing data in accordance withcertain aspects of the present disclosure. In one aspect, the method1101 may be implemented on a gateway such as gateway 610 of FIG. 6. Atstep 1105, the gateway 610 transmits a request to a sensor, such assensors 501, 550, for data corresponding to a period of time. In oneaspect, step 1105 may be performed periodically or in response to aparticular stimulus such as an asynchronous request or a userinteraction. In another aspect, the sensor 501 may be configured toperiodically or aperiodically transmit data to the gateway 610 withoutreceiving a request. Proceeding to step 1110, the gateway receives afirst set of samples from the sensor 501. As described above, the firstset of samples may comprise a set of k+δ CS samples taken by the sensor501. Proceeding to step 1115, the gateway 610 generates a second set ofsamples based on the first set of samples from the sensor 501. In oneaspect, the second set of samples corresponds to a complete set ofNyquist samples over the period of time. The process of generating thecomplete set of Nyquist samples from the CS samples is described ingreater detail below with respect to FIGS. 13-18. Proceeding to step1120, the gateway 610 performs additional processing on the second setof samples. For example, the gateway 610 may mitigate packet loss,remove artifacts in the received sample set, determine and mitigateclock misalignment between the gateway 610 and the sensor 501, andanalyze the second set of samples to interpret vital signs and triggeralarms. In another aspect the gateway may be configured to receive a setof samples from more than one sensor, each set corresponding to a commontime period. In some aspects, each set of samples may be aligned by thegateway 610 to facilitate comparison of the sensor inputs during thecommon time period. Advantageously, by performing CS reconstruction atthe gateway 610, the circuitry and power consumption at the sensor 510may be significantly reduced.

FIG. 12A illustrates a method 1201 for handling clock misalignment inaccordance with certain aspects of the present disclosure. In oneaspect, the method 1201 may be implemented on a gateway such as gateway610 of FIG. 6. In some communication systems, when there is a clockmismatch between sender and receiver, sample insertion/deletion at oneof the nodes may be performed to keep real time buffers from runningempty or overflowing. If proper care is not taken with sample insertionand deletion, discontinuities resulting from this operation may lead toundesirable artifacts. The complexity and the cost of a device depend onthe accuracy of the desired clock. For instance, a device synchronizedto the GPS clock will be more expensive than a device operating with aclock based on a ±20 parts per million (ppm) crystal. Similarly, adevice operating with a clock based on a ±75 ppm crystal will be furthereconomical. In the gateway 610 with CS reconstruction, Nyquist samplesare reconstructed at the gateway based on measurements at the sensor. Insome aspects, the gateway 610 typically has better clock management andcloser to the real wall-clock (such as GPS). This will enable thegateway 610 to synchronize streams of data from multiple sensors, evenwhen the number of measurements is k±δ, where δ comprises the number ofmeasurements due to clock mismatch. In some embodiments, the δ value maybe different for one or more of the multiple sensors. In addition, the δvalue may change over time for each sensor. Regardless, as describedherein, the receiver with CS reconstruction is capable of coping withvarying number of measurements for a given window of observation. Thereceiver may accomplish this by reconstructing N Nyquist samples foreach of the sensors 501, 550, where N corresponds to the window ofobservation based on the clock at the gateway.

This clock mismatch coping ability of CS reconstruction and itsadvantages are described further with respect to FIGS. 19A-C and 20A-C.FIG. 19A illustrates a waveform 1903 as detected at asensor/transmitter. As shown in the figure, the clock rate of thetransmitter is 10.1523 MHz. As illustrated a significant component ofthe waveform 1903 is present at 37 Hz. FIG. 19B illustrates areconstruction 1905 of the waveform 1903 generated at thereceiver/gateway. As shown, the clock rate at the receiver is 10 MHz.Accordingly, there is a mismatch of clock rates between the transmitterand receiver. As illustrated, in FIG. 19B, the clock mismatch betweenthe transmitter and the receiver has minimal effect on thereconstruction using CS techniques. Thus, the reconstructed waveform1905 is substantially similar to the waveform 1903 of FIG. 19A despitethe clock mismatch. As described above, the samples used in CSreconstruction may not be regularly spaced over the period of thesampled signal. This non-uniform sampling facilitates, in part, theaccurate reconstruction despite clock mismatch. FIG. 19C illustratesanother reconstruction 1907 of the waveform 1903 of FIG. 19A. As withthe waveform 1905 of FIG. 19B, the clock rate of the receiver/gateway is10 MHz. Thus, the same clock mismatch is present. However, thereconstruction 1907 is generated using traditional interpolation basedreconstruction. As shown, interpolation based reconstruction under theconditions of clock mismatch results in spurious tones 1909 in thereconstruction 1907. In part, the spurious tones may result fromtraditional clock mismatch mitigation. For example, one techniqueinvolves deleting two samples and inserting a sample linearlyinterpolated with its neighbors to maintain the receiver buffer fromover flowing because of the mismatch. Advantageously, using CStechniques, such mitigation schemes are unnecessary. Thus, accuratereconstruction can occur despite clock mismatch conditions. As notedabove, this also enables simpler, less power intensive clock circuitryto be used at the sensor without negatively affecting the data receivedat the gateway.

FIGS. 20A-C are similar to FIGS. 19A-C, but illustrate reconstructionswhere the transmitter/sensor has a clock rate that is misaligned fromand slower than the clock rate of the receiver/gateway. FIG. 20Aillustrates a waveform 2003 as detected at a sensor/transmitter. Asshown in the figure, the clock rate of the transmitter is 9.85 MHz. Asillustrated a significant component of the waveform 2003 is present at37 Hz. FIG. 20B illustrates a reconstruction 2005 of the waveform 2003generated at the receiver/gateway. As shown, the clock rate at thereceiver is 10 MHz. Accordingly, there is a mismatch of clock ratesbetween the transmitter and receiver. As illustrated, in FIG. 20B, theclock mismatch between the transmitter and the receiver has minimaleffect on the reconstruction using CS techniques. Thus, thereconstructed waveform 2005 is substantially similar to the waveform2003 of FIG. 20A despite the clock mismatch. As described above, thesamples used in CS reconstruction may not be regularly spaced over theperiod of the sampled signal. This non-uniform sampling facilitates, inpart, the accurate reconstruction despite clock mismatch. FIG. 20Cillustrates another reconstruction 2007 of the waveform 2003 of FIG.20A. As with the waveform 2005 of FIG. 20B, the clock rate of thereceiver/gateway is 10 MHz. Thus, the same clock mismatch is present.However, the reconstruction 2007 is generated using traditionalinterpolation based reconstruction. As shown, interpolation basedreconstruction under the conditions of clock mismatch results inspurious tones 2009 in the reconstruction 2007. In part, the spurioustones may result from traditional clock mismatch mitigation. Forexample, one technique involves inserting a sample linearly interpolatedwith its neighbors to maintain the receiver buffer from under flowingbecause of the mismatch. Advantageously, using CS techniques, suchmitigation schemes are unnecessary. Thus, accurate reconstruction canoccur despite clock mismatch conditions. As noted above, this alsoenables simpler, less power intensive clock circuitry to be used at thesensor without negatively affecting the data received at the gateway.

As noted above, the gateway 610 may have superior clock managementcompared with clock management on low power sensors 501 and 550. Thegateway 610 can send messages to the sensors, 501 and 550 that may beperiodic or aperiodic, causing them to synchronize them to the clock atgateway 610. This is illustrated in method 1201. In step 1205, thegateway 610 detects a misalignment between the clock of the gateway 610and the clock of the sensor 501. Proceeding to step 1210, gateway 610may transmit a clock synchronization signal to the sensor 501. In someaspects, step 1210 may be performed responsive to the determination instep 1205. In other aspects, step 1201 may be performed periodically orin response to other stimuli. In another aspect, gateway 610 may be incommunication with a plurality of sensors 501 and 550. Gateway 610 maybroadcast a clock synchronization signal to some or all of the pluralityof sensors 501 and 550.

Advantageously, the aspects described with respect to FIGS. 11 and 12Amay be combined. Thus, for example, the gateway 610 may maintain its ownfirst clock according to relatively precise circuitry. Similarly, thesensor 501 may maintain its own second clock according to relativelyimprecise circuitry. As described above, the sensor 501 generates a setof samples, such as CS samples, according to its second clock signal andtransmits the set to the gateway 610. Similarly, the gateway 610 maygenerate a second set of samples, such as a full set of Nyquist samples,according to its own first clock based on the received set of samples.Either periodically, in response to detection to misalignments betweenthe clocks, in response to requests from the sensor 501, or in responseto other stimuli, the gateway 610 may transmit a clock synchronizationsignal to the sensor 501.

FIG. 12B illustrates a method 1250 for handling clock misalignment inaccordance with certain aspects of the present disclosure. In oneaspect, method 1250 is implemented in a sensor such as the sensor 501 inFIG. 5. In step 1255 the sensor receives a clock synchronization signalfrom a gateway such as the gateway 610. Proceeding to step 1260, thesensor 501 synchronizes its clock to the clock of gateway 1260.

FIG. 13 illustrates an example block diagram for sensing andreconstruction of biomedical signals. A sensor 2602 for acquiringbiomedical signals, such as a PPG signal, may comprise three maincomponents: LEDs 2606, a photo-detector 2610, and lighting and samplingsequence for LED and photo-detector 2604 and 2612, respectively. TheLEDs 2606 may emit light with wavelengths between 600 nm and 1000 nm,which also comprises red and infrared parts of the spectrum. The lightfrom the LEDs 2606 may be transmitted/reflected from a tissue 2608(e.g., a human finger or ear, as illustrated in FIG. 13) and may becollected on the photo-detector 2610. The ratio of the averageintensities corresponding to the LEDs measured at the photo-detector maybe useful in determining the oxygen content (S_(p)O₂) in blood. Thus,S_(p)O₂ may be a function of the mean (DC content) of the PPG signal.

The lighting sequence 2604 and the sampling sequence 2612 may beobtained using a random seed generated by a seed generator 2614according to a defined under-sampling ratio (USR) 2616. The sampled data2618 from the photo-detector 2610 may be sent for Media AccessControl/Physical Layer (MAC/PHY) processing 2620 before transmission.The processed samples may be then packetized and transmitted by one ormore antennas 2622.

At a receiver side, as illustrated in FIG. 13, the transmitted samplesmay be received at one or more antennas 2624 and processed by MAC/PHYblock 2626. The data may be then passed to a re-constructor 2628 forobtaining the biomedical signal at Nyquist rate. For the accuratereconstruction, a random seed generator 2630 that generates a samplingsequence 2632 may need to be synchronized with the random seed generator2614 of the sensor.

In one aspect of the present disclosure, a gradient based sparsereconstruction 2636 may be applied on sampled data 2634 by using, forexample, a modified-Gabor sparsity basis regularizing vector for l₁-norm2638. An estimated signal 2640 may be then utilized by unit 2642 fortask-specific processing in order to obtain, for example, a bloodpressure estimate, a level of the oxygen in blood, and a heart rate. Thereconstructed data from other sensors such as 550 comprising ECG data or3D-accelerometer data may be available to 2642 for such task specificprocessing. An actual signal 2644 may be compared with a target signal2646 by unit 2648 in order to update USR utilized for generatingsampling instances. An updated USR value 2652 at the output of unit 2650may be used by the random seed generator 2630, and may be also fed backto the sensor for adapting the USR 2616 of the sensor. Beside the USR,additional feedback information may be also transmitted to the sensorfor adapting some other parameters, such as: a number of measurements atthe sensor, coefficients of a measurement matrix, a number oftransmitted samples of the signal, and a number of samples in eachtransmitted packet.

Modulations in the PPG waveform (associated with either red or infraredLEDs) may be related to the instantaneous blood flow. Instantaneousheart rate (HR) may be estimated as the inverse of the distance betweenwaveform peaks. The lighting sequence for the LEDs may depend upon thedesired sampling rate for the PPG signal. It may be assumed a uniformNyquist sampling rate. Also, it can be noted that frequent lighting ofthe LEDs may result in significant power consumption of the pulseoximeter sensor.

Certain aspects of the present disclosure support exploiting the sparsenature of the PPG signal and making fewer measurements in order to savethe sensor power. A Gabor basis may be employed as the transform spaceconsisting of various cosine waves with time support limited by Gaussianwindow functions at different scales.

Let the original sampled PPG signal be denoted by N-dimensional vector xand the sparse-domain transform basis be represented by N×N matrix W.The (i, j) entry of matrix W may be given as:

$\begin{matrix}{\lbrack W\rbrack_{i,j} = {{{\cos\left( \frac{2{\pi\left( {i - 1} \right)}\left( {j - 1} \right)}{2N} \right)} \times {\exp\left( {- \frac{\left( {i - 1} \right)^{2}\left( {j - {N/2}} \right)^{2}}{{wN}^{2}}} \right)}}.}} & (1)\end{matrix}$

The term w from equation (1) may be associated with the width of theGaussian kernel in the Gabor basis. Each row of the matrix W may benormalized such that the corresponding l₂-norm is equal to 1, and thematrix W may be referred as the sparse-basis. The PPG signal x may beprojected on the sparse-basis to generate the correspondingN-dimensional representation in the Gabor transform space and it may begiven as:y=W·x.  (2)

FIG. 14A illustrates an example of a short segment of a PPG signal and acorresponding representation y in the Gabor transform space inaccordance with certain aspects of the present disclosure. FIG. 14Billustrates the eight-second segment sampled at 125 Hz (i.e., a total ofN=1000 samples). It can be observed that the signal x may be sparse andcompressible in the transform domain with about 30 coefficients greaterthan 0.2 in terms of absolute magnitude. This indicates that most of thePPG signal characteristics may reside in a much lower-dimensional spacecompared to N and thus the PPG signal may be compressible.

Therefore, the CS principles may be exploited which allows to make K<<Nmeasurements (i.e. to heavily under-sample the original data) and stillbe able to estimate x with a high fidelity. If the signal x isexplicitly-sparse with only M non-zero elements in the transform space,then selecting K≥M log N/M samples at random from x may providesufficient information with a high probability to enable signalreconstruction with zero error.

In real situations, the signal may never be truly sparse and someinformation content may exist throughout the transform-space. However,the number of significant components with magnitude greater than ε,where ε<<max(y), may be much smaller than N. In FIG. 14, value of ε is0.2. This approach may be extended to the case where x is notexplicitly-sparse, and the CS paradigm may still remain valid. However,the reconstruction error may not be exactly equal to zero.

The sensing process for x may be mathematically expressed. Let P denotea K-dimensional vector containing unique entries (for example, chosen atrandom) with each element bounded between 1 and N. This may essentiallyprovide K random locations to select the elements from x. The seed forrandom number generation for constructing the vector P may be generatedlocally at the sensor or at the receiver. The seed may be based on keysused in security protocols of the communication link. The K-dimensionalmeasurement vector r, obtained from x, may be written as:r=H·x,  (3)where H denotes the K×N measurement matrix.

The i^(th) row of the matrix H from equation (3) may be an all-zerovector with 1 at the location given by the i^(th) element of P. It canbe noted that in the CS framework the measurement matrix may be definedas a matrix containing random independent and identically distributed(i.i.d.) elements. Such a measurement matrix may be necessary when it isnot known a priori that the input signal is sparse in the time domain orin the transform domain. In practice, the sensing process may beimplemented as Δ_(i)=MINDUR*USR+J(i), where Δ_(i) is the durationbetween (i−1)^(th) and i^(th) sampling instances, MINDUR is the minimumduration with uniform Nyquist sampling, USR is the under sampling ratio,and J(i) is some random jitter introduced for the i^(th) sample suchthat J(i)≤MINDUR.

The matching pursuit (MP) algorithm may be employed for signalreconstruction from the measurement vector r. The MP techniquerepresents a greedy algorithm that builds up a signal approximationiteratively by making a locally optimal decision. An initialization ofthe MP algorithm may be given by defining a modified basis V=H·W ofdimension K×N such that V=[V₁ . . . V_(N)], where V_(j) is the j^(th)column vector of V. Then, the residual may be initialized as r₀=r, withan approximation that ŷ=0. The dimensionality of vector ŷ is the same asthe dimensionality of y (i.e., N). The iteration counter may be alsoinitialized as i=1.

After that, the column vector from V may be found that maximizes theinner-product of the residual r_(i-1) onto V:

$\begin{matrix}{n_{i} = {\arg{\max\limits_{j = {1\mspace{14mu}\ldots\mspace{14mu} N}}{\frac{\left\langle {r_{i - 1},V_{j}} \right\rangle}{{V_{j}}_{L_{2}}}.}}}} & (4)\end{matrix}$

Then, the residual may be updated, and the coefficient vector y may beestimated as follows:

$\begin{matrix}{{r_{i} = {r_{i - 1} - {\frac{\left\langle {r_{i - 1},V_{n_{i}}} \right\rangle}{{V_{n_{i}}}_{l_{2}}^{2}}V_{n_{i}}}}},} & (5) \\{{\hat{y}}_{n_{i}} = {{\hat{y}}_{n_{i}} + {\frac{\left\langle {r_{i - 1},V_{n_{i}}} \right\rangle}{{V_{n_{i}}}_{l_{2}}^{2}}.}}} & (6)\end{matrix}$

After that, the iteration counter i may be incremented, and

$\Delta_{i} = \frac{{r_{i}}_{l_{2}}}{{r}_{l_{2}}}$may be defined. If i<m and Δ_(i)>ε, then the algorithm step defined byequation (4) may be repeated. Otherwise, {tilde over (Δ)}=Δ_(i) and ĩ=iand the algorithm may proceed to the step defined by equations (5) and(6). Finally, the original estimates may be obtained as {circumflex over(x)}=W·ŷ.

The term m represents the upper-bound on the number of iterationsallowed for reconstruction, and the term ε defines the convergencecriterion. Intuition behind the MP algorithm is twofold. At eachiteration step, the algorithm may attempt to find the column of V thatmay be most strongly correlated with the residual of r, and then maysubtract the contribution of this column vector from r. This algorithmis greedy in nature because at each step it may estimate the mostdominant component of the original signal x in the projection space W.It can be also noted that the main complexity of the MP algorithm liesin equation (4) which may cost O(K·N) arithmetic operations for a singleiteration.

Certain aspects of the present disclosure use the gradient-projectionbased sparse reconstruction (GPSR) approach to obtain a reconstructedsignal from the measurement vector r. This approach may estimate theoriginal signal x by jointly minimizing a data fidelity term (i.e. l₂norm of the error) and the l₁-norm in a transform space (i.e. measure ofsparsity) under no constraints. It is proposed in the present disclosureto modify this optimization problem by using a weighted l₁-norm. Thereconstruction algorithm may be given as:

$\begin{matrix}{{{\min\limits_{x}{{{Hx} - r}}^{2}} + {\tau{\sum\limits_{i = 1}^{N}{{\lbrack f\rbrack_{i}\left\lbrack {W \cdot x} \right\rbrack}_{i}}}}},} & (7)\end{matrix}$where f is a N-dimensional vector providing the relative importance ofcoefficients in transform space for computing the measure of sparsity(i.e., l₁-norm). The quantity 2 is a non-negative parameter indicatingthe relative weight of l₂-norm and l₁-norm in the cost function. Theterms [f]_(i) and [W·x]_(i) denote the i^(th) element of vectors f and[W·x]_(i), respectively.

The i^(th) element of vector f may be given by:

$\begin{matrix}{{\lbrack f\rbrack_{i} = \frac{1}{\left\lbrack {W \cdot \overset{\_}{x}} \right\rbrack_{i} + \sigma}},} & (8)\end{matrix}$where σ is a small regularizing parameter. The quantity x represents theensemble mean of the original signal vector x, and may be estimated byaveraging training example vectors. A segment from the MIMIC databasemay be utilized to estimate x, which can be then excluded from theexperimental validations described below.

FIG. 15 illustrates example operations for under-sampled acquisition ata sensor and reconstruction at a receiver in accordance with certainaspects of the present disclosure. FIG. 16 illustrates exampleoperations 2800 for actuating and de-actuating a light source at thesensor. As described below, the operations illustrated in FIGS. 15 and16 may be related. Thus, for the purposes of explanation, FIGS. 15 and16 are described jointly herein. At step 2510 of FIG. 15, non-uniformsampling instances may be generated at the sensor according to a randomseed. At step 2520, samples of a signal may be sensed at least during aplurality of the non-uniform sampling instances. At step 2810 of FIG.16, the source at the sensor may be actuated, for example, by turning onone or more LEDs at least during the plurality of the generatednon-uniform sampling instances, and the sensor may be de-actuated, atstep 2820, at least between the plurality of the non-uniform samplinginstances.

The sensed samples of the signal may be then packetized to obtain atleast one packet of the sensed samples, and the obtained at least onepacket may be transmitted over a wireless channel. At step 2530 of FIG.15, samples of the signal may be received from the sensor at are-constructor. At step 2540, a set of non-uniform sampling instancesmay be determined at the re-constructor during which signal was sampledat the sensor according to said random seed. In one aspect, the seed fornon-uniform sampling sequence may be generated at the re-constructorbased on keys used in security protocols of a communication link betweenthe sensor and the re-constructor. In another aspect, the seed fornon-uniform sampling sequence may be determined at the sensor andconveyed to the re-constructor (i.e., to the receiver). In yet anotheraspect, the seed for non-uniform sampling sequence may be determined atthe receiver and conveyed to the sensor. At step 2550, the signal may bereconstructed from the received samples using the determined non-uniformsampling sequence according to, for example, the modified GPSR algorithmdefined by equations (7)-(8).

The non-uniform sampling instances may be synchronized with receivedsamples of the signal for accurate reconstruction of the signal.Information observed during the reconstruction (e.g., at least one of:coefficients related to the reconstructed signal, a number of droppedpackets during the transmission, a channel signal-to-noise ratio, or thevariables {tilde over (Δ)} and ĩ) may be utilized to adapt varioussensor parameters (e.g., the USR, the number of measurements K, thenumber of transmitted samples of the signal N, the number of samples ofthe signal in each transmitted packet P, and the measurement matrix H)by conveying the observed information to the sensor via a feedbackmechanism. Then, the non-uniform sampling instances may be adapted atthe sensor according to the received feedback information.

Some reconstruction examples generated based on the CS approach arepresented in this disclosure. The eight-second segment may be selectedfrom the MIMIC database sampled at 125 Hz (i.e., N=1000 samples). It canbe recalled that the number of CS samples is K that defines anunder-sampling ratio (USR) as N/K. FIG. 17 illustrates an example forthe CS-PPG signal reconstructions obtained for the USR of 10, 20 and 30with the upper bound on the number of MP iterations m equal to 500. Thecurve 1710 represents the uniformly sampled original signal and curves1720, 1730 and 1740 represent reconstructed signals for values of USR of10, 20 and 30, respectively. FIG. 18 illustrates an example of thesignal sensed at non-uniform sampling instances at the USR of 40. Thesampling instances are shown as vertical lines.

It can be observed from FIG. 17 that the signal integrity may be wellpreserved until the USR of 20, and may start degrading thereafter.However, it can be noted that the signal peak locations may be wellpreserved even with high USR (i.e., the USR value of 30). In this case,the LED power consumption (as a part of the PPG data acquisition) may besignificantly reduced by the factor of USR because the LEDs may belightened for much smaller duration, specifically for only T·f_(s)/USRseconds instead of T·f_(s) seconds.

In some aspects, one or more of the functions recited herein areimplemented in a circuit configured to perform the one ore morefunctions. Similarly, the means described above may comprise one or morecircuits configured to implement the functionality of the means. Forexample, FIG. 22 illustrates aspects of a wireless communication device2350. The wireless communication device may correspond to a gateway suchas the gateway 610 of FIG. 6. In one aspect, the communication device2250 comprises a circuit for transmitting 2253, a circuit for receiving2255, a circuit for generating 2257, and a circuit for maintaining 2259.The circuit for transmitting 2253 may correspond to the BAN Radio 613 orthe BAN Radio 613 in conjunction with the BAN MAC 616 and the processingsystem 619 of FIG. 6. The circuit for receiving 2255 may correspond tothe BAN Radio 613 or the BAN Radio 613 in conjunction with the BAN MAC616 and the processing system 619 of FIG. 6. The circuit for generating2257 may correspond to the processing system 619. The circuit formaintaining 2259 may correspond to the processing system 619.

FIG. 23 illustrates aspects of a wireless communication device 2315. Thewireless communication device may correspond to a sensor such as thesensor 501 of FIG. 5. In one aspect, the wireless communication device2315 comprises a circuit for receiving 2323, a circuit for adjusting2329, a circuit for generating 2319, a circuit for transmitting 2331, acircuit for obtaining 2317, a circuit for determining 2321, a circuitfor conditioning 2325, and a circuit for sampling 2327. The circuit forreceiving 2323 may correspond to the BAN Radio 514 of FIG. 5 of the BANRadio 514 in conjunction with the BAN MAC 516. The circuit for adjusting2329 may correspond to the processing system 505. The circuit forgenerating 2319 may correspond to the CS ADC 513 of FIG. 5 or to the CSADC 513 in conjunction with the BAN MAC 516 and the processing system505 of FIG. 5. The circuit for transmitting 2331 may correspond to theBAN Radio 514 of FIG. 5 or to the BAN Radio 514 in conjunction with theBAN MAC 516 of FIG. 5. The circuit for obtaining 2317 may correspond tothe LNA 511 of FIG. 5 or to the LNA 511 in conjunction with the DAC 508.The circuit for determining 2321 may correspond to the LNA 511 and CSADC 513 of FIG. 5. The circuit for conditioning 2325 may correspond tothe LNA 511 and CS ADC 513 of FIG. 5. The circuit for sampling 2327 maycorrespond to the CS ADC circuit 513 of FIG. 5.

The various operations of methods described above may be performed byany suitable means capable of performing the corresponding functions.The means may include various hardware and/or software component(s)and/or module(s), including, but not limited to a circuit, anapplication specific integrate circuit (ASIC), or processor.

As used herein, the term “determining” encompasses a wide variety ofactions. For example, “determining” may include calculating, computing,processing, deriving, investigating, looking up (e.g., looking up in atable, a database or another data structure), ascertaining and the like.Also, “determining” may include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” may include resolving, selecting, choosing, establishingand the like.

As used herein, a phrase referring to “at least one of” a list of itemsrefers to any combination of those items, including single members. Asan example, “at least one of: a, b, or c” is intended to cover: a, b, c,a-b, a-c, b-c, and a-b-c.

The various operations of methods described above may be performed byany suitable means capable of performing the operations, such as varioushardware and/or software component(s), circuits, and/or module(s).Generally, any operations illustrated in the Figures may be performed bycorresponding functional means capable of performing the operations.

The various illustrative logical blocks, modules and circuits describedin connection with the present disclosure may be implemented orperformed with a general purpose processor, a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array signal (FPGA) or other programmable logic device(PLD), discrete gate or transistor logic, discrete hardware componentsor any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any commercially available processor,controller, microcontroller or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, aplurality of DSP cores, one or more microprocessors in conjunction withone or more DSP cores, or any other such configuration.

The steps of a method or algorithm described in connection with thepresent disclosure may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in any form of storage medium that is knownin the art. Some examples of storage media that may be used includerandom access memory (RAM), read only memory (ROM), flash memory, EPROMmemory, EEPROM memory, registers, a hard disk, a removable disk, aCD-ROM, a signal, and so forth. A software module may comprise a singleinstruction, or many instructions, and may be distributed over severaldifferent code segments, among different programs, and across multiplestorage media. A storage medium may be coupled to a processor such thatthe processor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isspecified, the order and/or use of specific steps and/or actions may bemodified without departing from the scope of the claims.

The functions described may be implemented in hardware, software,firmware or any combination thereof. If implemented in software, thefunctions may be stored as one or more instructions on acomputer-readable medium. A storage media may be any available mediathat can be accessed by a computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, a signal, or any other medium that can be usedto carry or store desired program code in the form of instructions ordata structures and that can be accessed by a computer. Disk and disc,as used herein, include compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers.

Thus, certain aspects may comprise a computer program product forperforming the operations presented herein. For example, such a computerprogram product may comprise a computer readable medium havinginstructions stored (and/or encoded) thereon, the instructions beingexecutable by one or more processors to perform the operations describedherein. For certain aspects, the computer program product may includepackaging material.

Software or instructions may also be transmitted over a transmissionmedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition oftransmission medium.

Further, it should be appreciated that modules and/or other appropriatemeans for performing the methods and techniques described herein can bedownloaded and/or otherwise obtained by an access terminal and/or accesspoint as applicable. For example, such a device can be coupled to aserver to facilitate the transfer of means for performing the methodsdescribed herein. Alternatively, various methods described herein can beprovided via storage means (e.g., RAM, ROM, a physical storage mediumsuch as a compact disc (CD) or floppy disk, etc.), such that an accessterminal and/or access point can obtain the various methods uponcoupling or providing the storage means to the device. Moreover, anyother suitable technique for providing the methods and techniquesdescribed herein to a device can be utilized.

It is to be understood that the claims are not limited to the preciseconfiguration and components illustrated above. Various modifications,changes and variations may be made in the arrangement, operation anddetails of the methods and apparatus described above without departingfrom the scope of the claims.

A wireless device in the present disclosure may include variouscomponents that perform functions based on signals that are transmittedby or received at the wireless device. A wireless device may also referto a wearable wireless device. In some aspects the wearable wirelessdevice may comprise a wireless headset or a wireless watch. For example,a wireless headset may include a transducer adapted to provide audiooutput based on data received via a receiver. A wireless watch mayinclude a user interface adapted to provide an indication based on datareceived via a receiver. A wireless sensing device may include a sensoradapted to provide data to be transmitted via a transmitter.

A wireless device may communicate via one or more wireless communicationlinks that are based on or otherwise support any suitable wirelesscommunication technology. For example, in some aspects a wireless devicemay associate with a network. In some aspects the network may comprise apersonal area network (e.g., supporting a wireless coverage area on theorder of 30 meters) or a body area network (e.g., supporting a wirelesscoverage area on the order of 10 meters) implemented usingultra-wideband technology or some other suitable technology. In someaspects the network may comprise a local area network or a wide areanetwork. A wireless device may support or otherwise use one or more of avariety of wireless communication technologies, protocols, or standardssuch as, for example, CDMA, TDMA, OFDM, OFDMA, WiMAX, and Wi-Fi.Similarly, a wireless device may support or otherwise use one or more ofa variety of corresponding modulation or multiplexing schemes. Awireless device may thus include appropriate components (e.g., airinterfaces) to establish and communicate via one or more wirelesscommunication links using the above or other wireless communicationtechnologies. For example, a device may comprise a wireless transceiverwith associated transmitter and receiver components (e.g., transmitter210 or 302 and receiver 212 or 304) that may include various components(e.g., signal generators and signal processors) that facilitatecommunication over a wireless medium.

The teachings herein may be incorporated into (e g, implemented withinor performed by) a variety of apparatuses (e.g., devices). For example,one or more aspects taught herein may be incorporated into a phone(e.g., a cellular phone), a personal data assistant (“PDA”) or so-calledsmartphone, an entertainment device (e.g., a portable media device,including music and video players), a headset (e.g., headphones, anearpiece, etc.), a microphone, a medical sensing device (e.g., abiometric sensor, a heart rate monitor, a pedometer, an EKG device, asmart bandage, etc.), a user I/O device (e.g., a watch, a remotecontrol, a light switch, a keyboard, a mouse, etc.), an environmentsensing device (e.g., a tire pressure monitor), a monitoring device thatmay receive data from the medical or environment sensing device (e.g., adesktop, a mobile computer, etc.), a point-of-care device, a hearingaid, a set-top box, or any other suitable device. The monitoring devicemay also have access to data from different sensing devices viaconnection with a network.

These devices may have different power and data requirements. In someaspects, the teachings herein may be adapted for use in low powerapplications (e.g., through the use of an impulse-based signaling schemeand low duty cycle modes) and may support a variety of data ratesincluding relatively high data rates (e.g., through the use ofhigh-bandwidth pulses).

In some aspects a wireless device may comprise an access device (e.g.,an access point) for a communication system. Such an access device mayprovide, for example, connectivity to another network (e.g., a wide areanetwork such as the Internet or a cellular network) via a wired orwireless communication link. Accordingly, the access device may enableanother device (e.g., a wireless station) to access the other network orsome other functionality. In addition, it should be appreciated that oneor both of the devices may be portable or, in some cases, relativelynon-portable. Also, it should be appreciated that a wireless device alsomay be capable of transmitting and/or receiving information in anon-wireless manner (e.g., via a wired connection) via an appropriatecommunication interface.

The invention claimed is:
 1. A method of processing data, the methodcomprising: maintaining, at a first apparatus, a first clock signal,wherein the first apparatus comprises a gateway of a body area network(BAN); transmitting, from the first apparatus, a clock synchronizationsignal for synchronizing a second clock signal, at a second apparatus,to the first clock signal, wherein the second apparatus comprises asensor in the BAN; receiving, at the first apparatus, one or morepackets comprising a first set of samples indicative of an input over aperiod of time, the first set of samples corresponding to the secondclock signal, and wherein samples in the first set of samples are spacedat non-uniform time-instances over the period of time; and generating,at the first apparatus, a second set of samples representing the signalover the period of time based, at least in part, on the first set ofsamples, wherein the second set of samples corresponds to the firstclock signal, and wherein samples in the second set of samples arespaced at uniform time instances over the period of time.
 2. The methodof claim 1, wherein the first set of samples comprises k+δ samples,wherein k is a target number of samples and δ is a number of additionalor absent samples.
 3. The method of claim 2, further comprising:determining a mismatch amount to which the first and second clocksignals are not synchronized, wherein in the number of additional orabsent samples is based, at least in part, on the mismatch amount; andcomparing the mismatch amount to a threshold, wherein: the clocksynchronization signal is transmitted when the mismatch amount theamount exceeds the threshold.
 4. The method of claim 1, wherein theclock synchronization signal is transmitted periodically oraperiodically.
 5. The method of claim 1, wherein: the first set ofsamples comprises n samples, wherein n is less than a number of samplescorresponding to Nyquist rate sampling the signal over the period oftime; and the second set of samples comprises n samples, wherein n isgreater than or equal to a number of samples corresponding to Nyquistrate sampling the signal over the period of time.
 6. The method of claim1, further comprising: receiving a third set of samples, wherein thethird set of samples corresponds to a third clock signal, wherein thethird set of samples represents a second signal over the period of time,and wherein samples in the third set of samples are spaced atnon-uniform time instances over the period of time; and generating afourth set of samples representing the second signal over the period oftime based, at least in part, on the third set of samples, the fourthset of samples corresponding to the second clock signal, and whereinsamples in the fourth set of samples are spaced at uniform timeinstances over the period of time.
 7. A gateway of a body area network(BAN), comprising: means for maintaining a first clock signal; means fortransmitting a clock synchronization signal for synchronizing a secondclock signal, at a sensor in the BAN, to the first clock signal; meansfor receiving one or more packets comprising a first set of samplesindicative of an input over a period of time, the first set of samplescorresponding to the second clock signal, and wherein samples in thefirst set of samples are spaced at non-uniform time-instances over theperiod of time; and means for generating, at the gateway of the BAN, asecond set of samples representing the signal over the period of timebased, at least in part, on the first set of samples, wherein the secondset of samples corresponds to the first clock signal, and whereinsamples in the second set of samples are spaced at uniform timeinstances over the period of time.
 8. The gateway of claim 7, whereinthe first set of samples comprises k+δ samples, wherein k is a targetnumber of samples and δ is a number of additional or absent samples. 9.The gateway of claim 8, further comprising: means for determining amismatch amount to which the first and second clock signals are notsynchronized, wherein in the number of additional or absent samples isbased, at least in part, on the mismatch amount; and means for comparingthe mismatch amount to a threshold, wherein: the clock synchronizationsignal is transmitted when the mismatch amount the amount exceeds thethreshold.
 10. The gateway of claim 7, wherein the clock synchronizationsignal is transmitted periodically or aperiodically.
 11. The gateway ofclaim 7, wherein: the first set of samples comprises n samples, whereinn is less than a number of samples corresponding to Nyquist ratesampling the signal over the period of time; and the second set ofsamples comprises n samples, wherein n is greater than or equal to anumber of samples corresponding to Nyquist rate sampling the signal overthe period of time.
 12. The gateway of claim 7, further comprising:means for receiving a third set of samples, wherein the third set ofsamples corresponds to a third clock signal, wherein the third set ofsamples represents a second signal over the period of time, and whereinsamples in the third set of samples are spaced at non-uniform timeinstances over the period of time; and means for generating a fourth setof samples representing the second signal over the period of time based,at least in part, on the third set of samples, the fourth set of samplescorresponding to the second clock signal, and wherein samples in thefourth set of samples are spaced at uniform time instances over theperiod of time.
 13. A gateway of a body area network (BAN), comprising:at least one processor coupled with a memory and configured to maintaina first clock signal; a transmitter configured to transmit a clocksynchronization signal for synchronizing a second clock signal, at asensor in the BAN, to the first clock signal; and a receiver configuredto receive one or more packets comprising a first set of samplesindicative of an input over a period of time, the first set of samplescorresponding to the second clock signal, wherein: samples in the firstset of samples are spaced at non-uniform time-instances over the periodof time; the at least one processor is further configured to generate asecond set of samples representing the signal over the period of timebased, at least in part, on the first set of samples; the second set ofsamples corresponds to the first clock signal; and samples in the secondset of samples are spaced at uniform time instances over the period oftime.
 14. The gateway of claim 13, wherein the first set of samplescomprises k+δ samples, wherein k is a target number of samples and δ isa number of additional or absent samples.
 15. The gateway of claim 14,wherein: the at least one processor is further configured to: determinea mismatch amount to which the first and second clock signals are notsynchronized, wherein in the number of additional or absent samples isbased, at least in part, on the mismatch amount; and compare themismatch amount to a threshold; and the transmitter is configured totransmit the clock synchronization signal when the mismatch amount theamount exceeds the threshold.
 16. The gateway of claim 13, whereintransmitter is configured to transmit the clock synchronization signalperiodically or aperiodically.
 17. The gateway of claim 13, wherein: thefirst set of samples comprises n samples, wherein n is less than anumber of samples corresponding to Nyquist rate sampling the signal overthe period of time; and the second set of samples comprises n samples,wherein n is greater than or equal to a number of samples correspondingto Nyquist rate sampling the signal over the period of time.
 18. Thegateway of claim 13, wherein: the receiver is further configured toreceive a third set of samples, the third set of samples correspondingto a third clock signal, the third set of samples representing a secondsignal over the period of time, and the third set of samples beingspaced at non-uniform time instances over the period of time; and the atleast one processor is further configured to generate a fourth set ofsamples representing the second signal over the period of time based, atleast in part, on the third set of samples, the fourth set of samplescorresponding to the second clock signal, and samples in the fourth setof samples being spaced at uniform time instances over the period oftime.